Bug 1572610 - Change xpcshell android_version to string type; r=jmaher

Change the xpcshell android_version to string type, consistent with mochitest:
https://searchfox.org/mozilla-central/rev/9775cca0a10a9b5c5f4e15c8f7b3eff5bf91bbd0/testing/mochitest/runtestsremote.py#102
test_backgroundfilesaver.js is the only xpcshell test that uses android_version; once bug 1572434
lands, that skip-if will not be required, so I am removing it here. (I'll be sure to rebase
and land on top of 1572434.)

Differential Revision: https://phabricator.services.mozilla.com/D41312

netwerk/test/unit/xpcshell.ini

@@ -98,7 +98,6 @@ requesttimeoutfactor = 2
 [test_authpromptwrapper.js]
 [test_auth_dialog_permission.js]
 [test_backgroundfilesaver.js]
-skip-if = (android_version == '18' && debug) # bug 1375459
 [test_bug203271.js]
 [test_bug248970_cache.js]
 [test_bug248970_cookie.js]

testing/xpcshell/remotexpcshelltests.py

@@ -244,7 +244,7 @@ def __init__(self, options, log):
         self.remoteTestRoot = posixpath.join(self.device.test_root, "xpc")
         # Add Android version (SDK level) to mozinfo so that manifest entries
         # can be conditional on android_version.
-        mozinfo.info['android_version'] = self.device.version
+        mozinfo.info['android_version'] = str(self.device.version)
 
         self.localBin = options['localBin']
         self.pathMapping = []